Electrolytic plate stack cell

ABSTRACT

A stacked plate cell having serially connected stacked electrodes is described, at least one stacked electrode consisting of a graphite felt plate, a carbon felt plate, a web having a carbon-covered starting material contact surface or a porous solid having a carbon-covered starting material contact surface or comprising such a material.

The present invention relates to a novel stacked plate cell and to aprocess for the electrolysis of substances.

Electrolysis cells are employed in modem chemistry in a variety of formsfor a multiplicity of tasks. An overview on the construction possibilities of electrolysis cells is found, for example, in D. Pletcher, F.Walsh, Industrial Electrochemistry, 2nd Edition, 1990, London, pp. 60ff.

A frequently used form of electrolysis cells is the stacked plate cell.A simple arrangement thereof is the capillary gap cell. The electrodesand corresponding separating elements are frequently arranged here likea filter press. In this type of cell, several electrode plates arearranged parallel to one another and separated by separating media suchas spacers or diaphragms. The intermediate spaces are filled with one ormore electrolyte phases. An undivided cell usually comprises only oneelectrolyte phase; a divided cell has two or more such phases. As arule, the phases adjacent to the electrodes are liquid. However, solidelectrolytes such as ion exchange membranes can also be employed aselectrolyte phases. If the electrode in this case is directly applied tothe ion exchange membrane, e.g. in the form of an electrocatalytic andfinely porous layer, additional contacts are necessary which, on the onehand, must be designed as current collectors and, on the other hand, assubstance transport promoters. The individual electrodes can beconnected in parallel (monopolar) or serially (bipolar). In the contextof the invention, cells having bipolar connection of the stackedelectrodes are exclusively considered.

In order to achieve as high a substance conversion as possible inelectrolysis cells, according to general knowledge the electrolyteshould be passed over the electrodes in such a way that optimumsubstance transport is achieved. In the case of liquid electrolytes, itis frequently proposed to allow the electrolyte liquid to flow parallelto the electrodes.

The space-time yield and the selectivity of the electrolysis alsodepend, in addition to the flow over the electrodes, on the electrodematerials used. These affect the service life, size and weight of thecell considerably.

In known stacked plate cells, the electrodes are as a rule designed assolid plates, for example graphite disks. Electrodes of this type havevarious disadvantages which result from the solidity of the material,for example the decreased surface area compared with a porous materialand the decreased substance conversion, higher weight and greater spacerequirement accompanying it.

It is thus an object of the present invention to provide a stacked platecell having increased space-time yield, high selectivity, low weight andspace requirement, which is as simple as possible to produce and tooperate. A further object of the invention is the provision ofelectrolysis processes having a high space-time yield and a highselectivity.

We have found that these objects are achieved by the stacked plate celldescribed in the claims and the processes described.

In the context of the invention, a stacked plate cell having serially(bipolar) connected stacked electrodes is provided, at least one stackedelectrode consisting of a graphite felt plate, a carbon felt plate, aweb having a carbon-covered starting material contact surface or aporous solid having a carbon-covered starting material contact surfaceor comprising such a material.

Felts suitable for use in the context of the present invention arecommercially available. Both graphite felts and carbon felts can beemployed here, both types of felt differing, especially, by thestructure of the carbon. Instead of or in addition to the feltsdescribed, other porous materials can also be used whose contactsurfaces with the starting material are completely or largely coveredwith carbon. Contact surfaces are in this case those external andinternal surfaces with which the starting material to be electrolyzedcomes into contact during the electrolysis reaction. These materials canin this case consist completely of carbon, for example carbon web,carbon gauzes or porous carbon solids. However, supports made of othermaterials can also be used whose contact surface with the startingmaterial is completely or mainly covered with carbon.

The electrode can be made entirely from the materials mentioned or haveone or more further layers. These layers can be used, for example, tostabilize the arrangement.

Preferably, the stacked plate cell, in particular the electrodesthemselves and the electrolyte, is designed such that as few aspossible, in the ideal case no electrolyte ions migrate through thecarbon-containing stacked electrode according to the invention describedabove on account of the electrical potential drop. The current withinthe electrode should if possible be caused exclusively by electrons, notby ions. Depending on the given electrolysis conditions, in particularthe electrolyte used, it may even be necessary to restrict or tosuppress this migration of electrolyte ions through thecarbon-containing stacked electrodes in order to achieve an appreciableelectrolysis reaction on these stacked electrodes.

This can be achieved by surrounding the carbon-containing stackedelectrode described above by a solid electrolyte. The solid electrolyteused can be fundamentally any material known for this function. Ionexchange membranes are preferably employed.

In this case, in addition to the solid electrolyte, a liquid electrolytephase which contains the electrolysis starting materials is also used.This liquid phase preferably contains no free conductive ions or onlysmall amounts thereof. An electronic current is thereby achievedexclusively or almost exclusively in the electrode. The ionic currentbetween the electrodes is then completely or largely represented by ionswhich are bonded in the solid electrolyte, i.e. do not move through thecarbon-containing stacked electrode freely on account of the potentialdrop.

Electrolyte liquids which are suitable for use in addition to solidelectrolytes contain less than 10% by weight of conducting salts,preferably less than 3% by weight. Preferred solvents are organicsubstances such as methanol, ethanol, DMF, acetic acid, formic acid oracetonitrile.

The stacked electrodes can also be separated from one another byelectrolyte-filled solids. An electrolyte-filled solid which can beused, in particular, is an electrolyte-filled web or gauze or adiaphragm.

The suppression of electrolyte ion migration according to the potentialdrop through the stacked electrode can in this case be hindered orsuppressed by the carbon-containing stacked electrode described abovecomprising an additional layer hindering or preventing the migration ofthe electrolyte ions through this electrode according to the potentialdrop. This layer preferably consists of graphite board. However, metalfoils can also be employed. These measures can be taken independently ofthe composition of the electrolyte, i.e. also additionally to a solidelectrolyte.

However, it is also possible to design the pore size or permeability ofthe stacked electrode, e.g. by impregnation, such that the electrolyteions, if possible, are not let through at all.

The stacked plate cells according to the invention offer an increasedsubstance conversion and an improved selectivity. In addition, thesestacked cells take up only about 20% to 70% of the stacking space ofconventional graphite stacked plate cells. The space saving is naturallyalso associated with a corresponding weight saving. In the cellsaccording to the invention, the incident flow on the individualelectrodes plays only a subordinate part. Expensive measures forimproving the substance transport to the electrodes can thus also bedispensed with without the space-time yield being adversely affected toa measurable extent.

The stacked plate cells described can be employed according to theinvention in electrolysis processes. An electrolysis process of thistype is suitable, in particular, for the oxidation of aromatics such assubstituted benzenes, substituted toluenes and substituted orunsubstituted naphthalenes. These substances are contained in the liquidelectrolyte phase of the stacked plate cell.

Processes for the methoxylation of 4-methoxytoluene, p-xylene,p-tert-butyltoluene, 2-methyl-naphthalene, anisole or hydroquinonedimethyl ether are particularly preferred. These substances can also beacyloxylated using the process according to the invention.

Another preferred process relates to the anodic dimerization ofsubstituted benzenes, substituted toluenes and substituted orunsubstituted naphthalenes, the substances mentioned preferably beingsubstituted by C₁ - to C₅ -alkyl chains. Advantageously, the processaccording to the invention can also be employed for the methoxylation orhydroxylation of carbonyl compounds, in particular of cyclohexanone,acetone, butanone or substituted benzophenones.

Another preferred process according to the invention is the oxidation ofalcohols or carbonyl compounds to carboxylic acids, e.g. of butynediolto acetylenedicarboxylic acid or of propargyl alcohol to propiolic acid.

The stacked plate cells according to the invention can advantageouslyalso be used for the functionalization of amides, in particular ofdimethylformamide to methoxymethyl-methylformamide.

The oxidation, reduction or functionalization of heterocycles using theprocess according to the invention described above is also advantageous.In this way, in particular, furan can be reacted to givedimethoxydihydrofuran or N-methylpyrrolid-2-one to give5-methoxy-N-methylpyrrolid-2-one.

EXAMPLES Example 1

Methoxylation of p-xylene

p-Xylene was methoxylated in a stacked plate cell according to theinvention. The electrolysis cell contained a stack of 6 annular disks ofgraphite felt type RVG 1000 from the company Deutsche Carbone having athickness of 3 mm, an internal diameter of 30 mm and an externaldiameter of 140 mm. As a support for the electrolyte phase, annulardisks of polypropylene filter gauzes having a thickness of 1.8 mm weremounted between the electrode plates. This cell was integrated in arecirculating apparatus in which the liquid electrolyte solution,consisting of a mixture of 450 g of p-xylene to be methoxylated, 30 g ofsodium benzenesulfonate, and also 2520 g of methanol, was recirculated.

The electrolysis was carried out at a temperature from approximately 30°C. to 40° C., a voltage of 5 V to 6 V and a current strength ofapproximately 5 A until an amount of current measured by the hydrogendevelopment on the cathode of 4.4 F per mole of p-xylene had beenemployed.

The substance conversion was 99% and the current yield 74% with a yieldof 71% of tolylaldehyde dimethyl acetal and 24% of tolyl methyl ether.

Example 2

Electrolysis of Cyclohexanone

The plate stack consisted of 12 annular disks of graphite felt of thetype RVG 2003 from the company Deutsche Carbone having a thickness of 3mm, an internal diameter of 30 mm and an external diameter of 140 mm.Between the plates was in each case arranged a 2 mm thick layer ofgraphite board of the type Sigraflex from the company Sigri and a filtergauze of polypropylene. These intermediate layers were likewiseconstructed as annular disks.

The electrolyte consisted of 600 g of cyclohexanone to be electrolyzed,2259 g of methanol, 66 g of water, 15 g of potassium iodide and 60 g ofpotassium hydroxide (43% strength).

The electrolysis temperature was from 15° C. to 20° C. and the currentstrength was approximately 5 A. The electrolysis was terminated after acharge transport of 2.2 F per mole of cyclohexanone.

A substance conversion of 97% was achieved. The yield of1-hydroxycyclohexan-2-one dimethyl ketal was 71%. This product wasobtained in pure form by distillation after distilling off the methanoland separating off the conductive salt. In this case, the iodine contentof the ketal was less than 1 ppm.

Comparison Example to Example 2

For comparison, cyclohexanone was treated in a conventional electrolysiscell having a plate stack of 11 annular disks. The annular disksconsisted of flat-ground solid graphite having an unevenness of lessthan 0.1 mm, and had a thickness of 5 mm, an internal diameter of 30 mmand an external diameter of 140 mm. The electrode disks were arranged inthe cell at a distance of 0.5 mm from one another, the plate distancebeing maintained by radially arranged polypropylene strips which coveredless than 10% of the electrode surface.

The liquid electrolyte solution consisted of a mixture of 675 g ofcyclohexanone to be electrolyzed, 1965 g of methanol, 45 g of water, 2 gof NaOCH₃ and 90 g of potassium iodide.

The electrolysis was carried out at a temperature from approximately 30°C. to 40° C. and a current strength of approximately 5 A until an amountof current of 2.2 F per mole of cyclohexanone had been employed.

The substance conversion was 98% with a distinctly lower yield of 62% of1-hydroxycyclohexan-2-one dimethyl ketal. After distilling off methanoland separating off the conductive salt, an iodine content ofapproximately 30 ppm is obtained in the distilled goods.

The electrolysis cell according to the invention thus allows distinctlyincreased yields together with comparable energy use with, at the sametime, lower use of potassium iodide, which can be replaced to aconsiderable extent by the more favorable potassium hydroxide. This inturn leads to a purer electrolysis product.

Example 3

Methoxyation of p-xylene

Construction and the carrying-out of the experiments corresponded toExample 1. Instead of pure graphite felt electrodes, however, electrodeswere used which were composed of a layer of graphite felt of the typeSigratherm GDF 5 from the company Sigri connected as the anode and of alayer of RA2 foil connected as the cathode.

The electrolysis was carried out at from 48° C. to 55° C. and at acurrent strength of approximately 5 A. It was terminated at a chargetransport of 7.5 F per mole of p-xylene. In this case, a yield of 86% oftolylaldehyde dimethyl acetal was achieved with a substance conversionof 99%.

Comparison Example to Example 3

Instead of the electrodes described above in Example 3, solid graphiteplate electrodes were used such as were described above in thecomparison example to Example 2. The electrolysis conditionscorresponded to those described in Example 3.

At a substance conversion of 99%, the yield of tolylaldehyde dimethylacetal was 77%. Even the modified electrode arrangement according to theinvention thus offers considerable advantages in the space-time yield ofthe electrolysis process.

Example 4

Methoxylation of Dimethyformamide (DMF)

In this electrolysis cell according to the invention, the plate stackconsisted of an alternating sequence of 9 annular disks of the type RVG1000 from the company Deutsche Carbone and 8 annular disks of the typeNafion 117 from the company Dupont, which were arranged as described inExample 1. The Nafion 117 was swollen in DMF at 110° C. for 10 minbeforehand.

The electrolyte liquid initially introduced into the apparatus contained584 g of DMF and 2560 g of methanol. The electrolysis temperature wasfrom 40° C. to 47° C., and the cell voltage was from 5 V to 6 V and thecurrent strength from 3 A to 5 A.

A conversion of DMF of approximately 90% was achieved. After the removalof methanol on a rotary evaporator, a (di)methoxy-DMF yield ofapproximately 70% was achieved. The selectivity was around 70%; it waspossible to achieve selectivities of almost 90% with only a slightlydecreased conversion.

In the continuous experiment, after a running time of 390 hours at anaverage current use of 1.66 F per mole of DMF an average selectivity of79% was achieved. The average current yield was just under 90% based onthe DMF consumption.

Comparison Example to Example 4

A conventional electrolys is cell was used, such as is described in thedissertation by R. Grege, Dortmund, 1990, pages 8 to 10. Theintermediate layer used between the electrodes was Nafion 117, which wasswollen in DMF at 110° C. for 10 min beforehand.

The electrolysis temperature was 80° C. The current yield was 95% andthe conversion of dimethyl-formamide only 10%.

An additional advantage of the cell according to the invention comparedwith the conventional cell described by way of example by Grege resultsfrom the simpler assembly and arrangement of the plate stack. Equipmentfor holding and adjusting the graphite plates is completely unnecessaryhere, as the felt plates are simply stacked alternately with solidelectrolytes. The stacked plate cell according to the invention is thusnot only lighter and smaller, but also significantly more easilyconstructed.

We claim:
 1. A stacked plate cell comprisinga plurality of seriallyconnected stacked electrodes comprising at least one carbon-containingstacked electrode comprising a graphite felt plate, a carbon felt plate,a web having a carbon-covered starting material contact surface or aporous solid having a carbon-covered starting material contact surface,a liquid electrolyte phase, and a barrier to hinder or prevent migrationof electrolyte ions on account of the electrical potential drop throughthe carbon-containing stacked electrode.
 2. A stacked plate cell asclaimed in claim 1, wherein the barrier comprises a solid electrolytetouching the carbon-containing stacked electrode.
 3. A stacked platecell as claimed in claim 2, wherein the solid electrolyte is an ionexchange membrane.
 4. A stacked plate cell as claimed in claim 1,wherein the liquid electrolyte phase contains no free conductive ions oronly small amounts of free conductive ions.
 5. A stacked plate cell asclaimed in claim 1, wherein at least two stacked electrodes areseparated by an electrolyte-filed solid.
 6. A stacked plate cell asclaimed in claim 5, wherein the electrolyte-filed solid is anelectrolyte-filled web or gauze or a diaphragm.
 7. A stacked plate cellas claimed in claim 1, wherein the carbon-containing stacked electrodecomprises a layer hindering or preventing the migration of theelectrolyte ions vertically through this stacked electrode.
 8. A stackedplate cell as claimed in claim 7, wherein the layer hindering orpreventing the migration of the electrolyte ions is made of graphiteboard.
 9. A stacked plate cell as claimed in claim 1, wherein thecarbon-containing stacked electrode itself serves as the barrier.
 10. Anelectrolysis process, comprisingpassing an electric current through astacked plate cell comprisinga plurality of serially connected stackedelectrodes comprising at least one carbon-containing stacked electrodecomprising a graphite felt plate, a carbon felt plate, a web having acarbon-covered starting material contact surface or a porous solidhaving a carbon-covered starting material contact surface, and a liquidelectrolyte phase, and hindering or preventing migration of electrolyteions on account of the electrical potential drop through thecarbon-containing stacked electrode.
 11. A process as claimed in claim10, wherein the liquid electrolyte phase in the stacked plate cellcomprises aromatic compounds, in particular substituted benzenes,substituted toluenes or substituted or unsubstituted naphthalenes, andthese are oxidized.
 12. A process as claimed in claim 10, wherein theliquid electrolyte phase in the stacked plate cell comprises4-methoxytoluene, p-xylene, p-tert-butyltoluene, 2-methylnaphthalene,anisole or hydroquinone dimethyl ether and these are alkoxylated oracyloxylated.
 13. A process as claimed in claim 10, wherein the liquidelectrolyte phase in the stacked plate cell comprises substitutedbenzenes, substituted toluenes or substituted or unsubstitutednaphthalenes and these are anodically dimerized, the aromaticspreferably being C₁ - to C₅ -alkyl-substituted.
 14. A process as claimedin claim 10, wherein the liquid electrolyte phase in the stacked platecell comprises carbonyl compounds, in particular cyclohexanone, acetone,butanone or substituted benzophenones, and these are methoxylated orhydroxylated.
 15. A process as claimed in claim 10, wherein the liquidelectrolyte phase in the stacked plate cell comprises alcohols orcarbonyl compounds and these are oxidized to carboxylic acids, e.g.butynediol to acetylenedicarboxylic acid or propargyl alcohol topropiolic acid.
 16. A process as claimed in claim 10, wherein the liquidelectrolyte phase in the stacked plate cell comprises amides and theseare functionalized, dimethylformamide, in particular, beingfunctionalized to methoxymethyl-methylformamide.
 17. A process asclaimed in claim 10, wherein the liquid electrolyte phase in the stackedplate cell comprises heterocycles and these are oxidized, reduced orfunctionalized, furan, in particular, being converted todimethoxydihydrofuran or N-methylpyrrolid-2-one to5-methoxy-N-methylpyrrolid-2-one.